With wide application of an intelligent terminal, a demand of people on a network traffic is increasing. In order to meet a demand of people on surfing an internet at any time and anywhere, performance of a network system should be improved. Therefore, great evolution has been made on a cellular network, for 3GPP (The 3rd Generation Partnership Project), from GSM (Global System for Mobile communications) to UMTS (Universal Mobile Telecommunications System), LTE (Long Term Evolution), and LTE-Advanced, and a Wi-Fi (wireless fidelity) system in a wireless local area network has evolved from IEEE 802.11a/b to 802.11g, 802.11n and 802.11ac.
At a same time, great progress has also been made in a field of sequence design, and the cellular network absorbs a research result in the field of the sequence design in time, and introduces a Zadoff-Chu sequence with a perfect autocorrelation property. However, the Wi-Fi system fails to follow up in time, and LTF (Long Training Field) still uses a pseudo-noise sequence as a basic sequence for generating the LTF.
Specifically, an existing long training sequence has the following several forms.
(1) In IEEE 802.11a and IEEE 802.11g, an expression of the long training sequence on a frequency domain is as follows:
L−26,26={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1, −1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}.
A structure of an OFDM training frame of 802.11g is as shown in FIG. 1, and the long training sequence occupies two OFDM symbols on a time domain, which is mainly used for improving precision of channel estimation.
(2) In IEEE 802.11n, under a 20 MHz bandwidth and a 40 MHz bandwidth, long training sequence symbols for high speed transmission are respectively:
HTLTF−28,28={1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1, 1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,−1,−1};
HTLTF−58,58={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1, −1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,−1,−1,−1,1,0,0,0,−1,1,1,−1,1,1,−1,−1,1,1,−1,1,−1, 1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1, 1,1};
where a format of the LTF in a high speed-greenfield frame HT-greenfield PPDU (Physical layer Protocol Data Unit) is as shown in FIG. 2.
(3) In EEE 802.11ac, expressions of VHTLTF in bandwidths of 20 MHz, 40 MHz, 80 MHz and 160 MHz are respectively shown as follows:
VHTLTF−28,28={1,1,LTFleft,0,LTFright,−1,−1}=HTLTF−28,28;
VHTLTF−58,58={LTFleft,1,LTFright,−1,−1,−1,1,0,0,0,−1,1,1,−1,LTFleft,1,LTFright}=HTLTF−58,58;
VHTLTF−122,122={LTRleft,1,LTFright,−1,−1,−1,1,1,−1,1,−1,1,1,−1, LTFleft,1,LTFright,1,−1,1,−1,0,0,0,1,−1,−1,1,LTFleft,1,LTFright,−1,−1,−1,1,1,−1,1,−1,1,1,−1,LTFleft,1,LTFright};
VHTLTF−250,250={VHTLTF−122,122,0,0,0,0,0,0,0,0,0,0,0, VHTLTF−122,122};
where sequences represented by LTFleft and LTFright are as follows:
LTFleft={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1};
LTFright={1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1, 1,1,1,1}.
A format of VHTLTF in VHT PPDU is as shown in FIG. 3.
At least the following problems exist in the prior art.
In an existing Wi-Fi system, all transmitting ends use a same long training sequence, and a receiving end may not distinguish the transmitting ends according to the long training sequence. After the receiving end receives a signal, no matter the signal is sent to itself or not, the receiving end needs to demodulate the signal, because the receiving end can not distinguish the transmitting ends in a physical layer and can only distinguish the transmitting ends in an MAC layer. Moreover, it can be seen from the above-mentioned autocorrelation property curve that, an autocorrelation property of an LTF sequence is not ideal and its ability to resist frequency offset is poor.